1. Field of Invention
The invention relates to the field of fluid powered actuators, in particular to an actuator for ejecting stores from an aircraft.
2. Description of Prior Art
The gravity release of stores from an aircraft flying at high speeds, and particularly supersonic speeds, is not practicable. The high velocity of the air and the shock waves produced by the wings and fuselage disrupt the trajectory of the store, reducing accuracy; and sometimes causing store to store or store to aircraft collisions. Thus, such aircraft usually incorporate ejector mechanisms in the stores racks so that, upon release, the store will be rapidly separated from the aircraft minimizing the effect of the airflow.
A typical prior art ejector system is disclosed in U.S. Pat. No. 3,883,097, Device for Picking Up and Ejecting Loads Under an Aircraft by Billot. Billot discloses a bomb rack with fore and aft ejector cylinders. The ejection pressure is provided by an explosive cartridge which fires into a central chamber. The gases are conducted from the chamber by passageways to each ejector cylinder. Thus, with the Billot design, the only way to change the ejection force is to change the power output of the cartridge. This is a serious shortcoming for it's important to have the capability to vary the ejection force as a function of the size and/or the weight of the store, and the particular maneuver used by the aircraft during delivery. Thus, in order to achieve this capability a large number of cartridges with different power outputs must be stocked. But this will only provide discrete changes in output force and the ejection force cannot be reprogrammed in flight. There is also no provision for controlling the gas pressure applied to each ejector so that the pitch angle of the store can be varied as it is ejected from the aircraft.
The pitch angle at which the store is ejected from the aircraft may also have to be varied as a function of the particular maneuver of the aircraft during delivery to achieve acceptable accuracies and safe separations. Furthermore, the gas pressure from the cartridge immediately rises to a high peak and then decays as a function of piston stroke and cooling of the explosive products, and thus the force applied to the store during ejection is non-linear. This makes the computation of the trajectory of the store more complicated than it would be if the force were constant, and requires a longer stroke in order to achieve acceptable separation velocities.
The advantage of having a constant ejection force becomes obvious when it is realized that force equals the mass times the acceleration (F=MA), thus, with a constant force the store will be ejected at a constant acceleration rate. This, of course, makes the computation of trajectories much simpler, and minimizes the stroke length, ejector reaction loads and stroke accelerations.
Another example is U.S. Pat. No. 3,610,094, Ejector Release Units for Use in Aircrafts by Craigie. Craigie discloses a stores rack with explosively actuated ejector pistons. Diverter valves are provided for controlling the gas pressure applied to each of the ejector pistons. While such a system can vary the pitch of the store by causing a differential pressure to exist in each ejector, there is no method disclosed to ensure that a constant force is applied to the store during ejection.
A further example of an ejector system can be found in U.S. Pat. No. 3,974,990, Dual Ejector Stores Attitude Control System by Holt, et al. Holt, et al. disclose a pneumatically actuated ejector mechanism. A pneumatic pressure source is used to drive a coupling piston into a liquid filled actuation cylinder containing an ejector piston. The actuation cylinder also incorporates a pressure relief piston. Upon actuation, pneumatic pressure drives the coupling piston into the liquid within the actuation cylinder compressing the relief piston. Thus, initially no force is applied to the ejector piston. After the relief piston has moved to its stop, pressure builds up against the ejector piston. Thus, not only can the overall force applied be controlled by setting the pneumatic pressure level, but the stroke of the ejector against each end of the store can be adjusted for pitch control. But, this system is very complicated and inefficient in that it does not provide a constant ejection force.
As previously mentioned, one of the problems of using explosively actuated cartridges as the power source is that very high initial peak pressures are generated followed by a rapid decay in pressures. Attempts to alleviate this problem have centered on changing the effective cross-sectional area of the ejector piston. For example, U.S. Pat. No. 4,049,222, Ejector Rack for Nuclear Stores by Peterson. In the Peterson design, at the start of the stroke, when gas pressure is high, the gas enters through a probe engaged with a small cavity inside the piston, and, thus, the probe effectively seals off a large portion of the piston area. As the piston starts to move downward, the increase in volume tends to reduce the pressure and, in order to compensate for this pressure reduction, the probe allows the gas to act on additional discrete areas of the top of the piston after predetermined increments of piston travel. But, Peterson does not disclose a method of varying the force for pitch control, nor does he disclose a method of varying the overall pressure level within the cylinder to accommodate different sizes and/or weights of stores.
Other systems for ejecting stores can be found in U.S. Pat. No. 3,009,730, Ejector for External Carriage Stores by E. K. Gantschnigs, et al., U.S. Pat. No. 3,810,671, Jettison Device for Helicopter Load Carrying System by Jeffrey, and U.S. Pat. No. 3,877,343, Stores Carrier by Newell, et al.
Of course, variable output force actuation cylinders are not new, of example, U.S. Pat. No. 1,081,690, Automatic Plunger Elevator by Morton. Morton discloses a hydraulically powered actuator cylinder for lifting an elevator which comprises a central piston encased in a sleeve. The piston and sleeve ride within an outer cylinder having an array of orifices. Fluid enters through the array of orifices and as the piston and sleeve rise, the sleeve uncovers additional orifices, increasing the flow rate into the cylinder, and, thus, the force applied to the piston. The disadvantage of this actuator is that because the piston rides within the sleeve, the piston area upon which pressure can be applied is necessarily small in relationship to the overall size of the actuator and, thus, the available output force to size and/or weight ratio is low. For this reason, and the fact that there is no provision for setting the level of the constant force output, it is unattractive for use as a stores ejector.
Of additional interest are U.S. Pat. No. 3,797,615, Impact Cushioning Device by Stembridge, and U.S. Pat. No. 3,731,770, Adjustable Shock Absorber by Binden. Both Stembridge and Binden disclose shock absorber cylinders which use similar mechanisms to vary resistance as a function of stroke. A piston is movably mounted within the cylinder having a bore which engages a first tube having a first array of orifices. Within the first tube is a second tube rotatably mounted therein, having a second array of orifices. Thus, when the piston rod is impacted, the piston moves in a direction so as to sequentially cover the first array of orifices, and the flow rate out of the cylinder into the second tube decreases as a function of stroke. Rotation of the second tube varies the overlap of the first arrays of orifices and, therefore, provides a method for varying the overall resistance to stroke. The functioning and purpose, therefore, of these two shock absorbers is exactly opposite to that desired of an actuator for ejecting a store.
Therefore, it is a primary object of this invention to provide an actuator with a pre-selected output force vs stroke profile.
It is another object of this invention to provide an actuator with a pre-selected output force vs stroke profile adapted to utilize a variable pressure level source.
A further object of this invention is to provide an actuator with a substantially constant output force vs stroke profile adapted to utilize a variable pressure level source.
A still further object of this invention is to provide an actuator wherein the magnitude of a pre-selected output force vs stroke profile is infinitely adjustable.